THE MICROSCOPE 133 



relative to the optical axis can be altered. In order to 

 centre, a diaphragm with small aperture is used, and the 

 hole in the diaphragm is focussed with a low power ; then, 

 by means of the lateral screws, this hole is brought into 

 the centre of the field. Below the sub-stage condenser 

 a mirror with concave and plane surfaces should be fitted, 

 the plane surface being used with the condenser, as a 



FIG. 23. Swift's detachable mechanical stage. 



general rule. The concave mirror may be used for illumi- 

 nation with low-power objectives, the condenser being 

 detached or swung out of position. The necessity for 

 careful illumination must be insisted upon ; in fact, to 

 obtain the best results the light should be readjusted for 

 every specimen by mirror, diaphragm, and condenser, 

 i.e. " critical " illumination should be aimed at. A good 

 specimen may be utterly spoilt, visually, by faulty illumi- 

 nation ; while an indifferent one may be made to look 

 passable by proper illumination. In the examination of 

 micro-organisms in the fresh or living and unstained 

 condition, it is necessary, as a rule, to diminish the light 

 by means of a small diaphragm, or by racking down the 

 condenser, or by both ; while for stained or opaque objects 



